smc911x: Fix build warnings

Commit 6af1d41 "smc911x MII made available" was missing a few "const"
qualifiers.  Fix the resulting in build warnings:

smc911x.c: In function 'smc911x_initialize':
smc911x.c:297: warning: passing argument 2 of 'miiphy_register' from incompatible pointer type
smc911x.c:297: warning: passing argument 3 of 'miiphy_register' from incompatible pointer type

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Helmut Raiger <helmut.raiger@hale.at>

drivers/net/smc911x.c

@@ -237,7 +237,7 @@ static int smc911x_rx(struct eth_device *dev)
 
 #if defined(CONFIG_MII) || defined(CONFIG_CMD_MII)
 /* wrapper for smc911x_eth_phy_read */
-static int smc911x_miiphy_read(char *devname, u8 phy, u8 reg, u16 *val)
+static int smc911x_miiphy_read(const char *devname, u8 phy, u8 reg, u16 *val)
 {
 	struct eth_device *dev = eth_get_dev_by_name(devname);
 	if (dev)
@@ -245,7 +245,7 @@ static int smc911x_miiphy_read(char *devname, u8 phy, u8 reg, u16 *val)
 	return -1;
 }
 /* wrapper for smc911x_eth_phy_write */
-static int smc911x_miiphy_write(char *devname, u8 phy, u8 reg, u16 val)
+static int smc911x_miiphy_write(const char *devname, u8 phy, u8 reg, u16 val)
 {
 	struct eth_device *dev = eth_get_dev_by_name(devname);
 	if (dev)